[cDAC] efficient lookup of constructed types

Currently the cDAC looks up constructed types as a linear search of the EEHashTable m_pAvailableTypeParams. This is not very efficient, so we will want to look at the performance and adjust the lookup mechanism accordingly.

Initial ideas
* Copy the runtime hash lookup algorithm
-- Simplify the lookup algorithm in the runtime?
-- If we choose this path, is there a way to minimize the complexity that we need in the cDAC?

* Store types in our own hash table
-- Already doing this to some extent with a cache of already-found items
-- Will have to deal with Flush()?